In these days, importance of a digital coherent optical communication technology is increasing, as a demand for a network with high speed and large capacity is increasing. A digital coherent reception system is also called an intradyne reception system. The digital coherent reception system can achieve receiving sensitivity improvement of 3 dB to 6 dB or more, compared with a modulation system, like OOK (on-off keying) and DPSK (differential quadrature phase shift keying). The digital coherent reception system advantageously has high compatibility with a multi-level modulation system, like a polarization multiplexing system and QAM (quadrature amplitude modulation).
FIG. 10 is a diagram illustrating a configuration of a digital coherent receiver 400 described in Patent Document 1 and Non-Patent Document 1. A signal light entering the digital coherent receiver is a multiplexed signal (DP-QPSK signal light) with 4 channels (Ix, Qx, Iy, Qy) using a Polarization multiplexing (DP: dual-polarization)-QPSK (quadrature phase shift keying) signal.
The DP-QPSK signal is separated to each channel of Ix, Qx, Iy, and Qy at a polarization diversity 90-degree hybrid 401. The separated signals are converted into analog electric signals per channel at respective optical-electric (O/E) converters 402-1 to 402-4. Respective O/E-converted signals are converted into digital signals at respective A/D (analog to digital) converters 403-1 to 403-4 which carry out sampling in synchronization with a reference sampling clock (CLK) 405.
The digital signals converted by the A/D converters 403-1 to 403-4 enter a digital signal processing circuit 404. A background of why the digital signal processing circuit 404 is employed in the digital coherent reception system and a function of the digital signal processing circuit 404 are described below.
The coherent receiver which does not carry out digital signal processing includes a problem that it is difficult to keep stable reception due to off-set of a frequency and a phase of a LO (local oscillator) light and polarization fluctuation.
In the meantime, as an electronic device technology improves, it has become possible to use a high-speed A/D converter for signal processing of a high-speed communication device. As a result, by carrying out digital signal processing for a signal converted to a digital signal, it has become possible to compensate off-set of a frequency and a phase of a LO light which has been a problem of the coherent reception system which does not carry out digital signal processing. The digital signal processing enables to compensate polarization fluctuation of an optical signal.
As described above, the digital coherent reception system achieves stable and accurate coherent reception compared with the coherent reception system which does not carry out digital signal processing.
Furthermore, in the digital coherent reception system, it is possible to add compensation of wavelength dispersion and high waveform equalization to a signal in addition to compensation of offset of a frequency and a phase and compensation of polarization fluctuation.
Non-Patent Document 2 describes a skew compensation technology that is a technology for compensating waveform distortion used in the digital coherent receiver. Non-Patent Document 2 describes the technology which achieves highly accurate skew compensation through quadratic function approximation on adjacent sampling points and sampling points before and behind the adjacent sampling points by using FIR (finite impulse response) filter.
Since a high-speed A/D converter becomes widespread, in backplane transmission which connects circuit substrates to each other, it becomes possible to conduct high-level waveform equalization and highly accurate digital clock extraction, like MLSE (most likelihood sequence estimation) using digital signal processing or the like, in order to tackle the problem of interference between codes and deterioration of jitter characteristics due to band shortage of a transmission path.